1. Field of the Invention
The present invention relates generally to signal reproducing systems for use in optical disc players, and more particularly, to a system for reproducing an information signal in an optical disc player which is operative to rotate an optical record disc on which the information signal is recorded and cause a light beam to impinge upon the optical record disc rotating at a predetermined speed so as to read the information signal from the optical record disc.
2. Description of the Prior Art
In an optical disc player for reproducing an information signal recorded on an optical record disc, an optical head device is provided for constituting an optical arrangement to read the information signal from a spiral track or concentrically circular record tracks formed on the optical record disc. An example of such an optical head device is arranged as shown schematically in FIG. 1.
The optical head device shown in FIG. 1 is formed into an optical unit 10 which contains a semiconductor laser element 1, an objective lens 6, a photodetector 8 and other optical parts and is movable in a direction of the radius of an optical record disc D on which, for example, a spiral record track is formed, as indicated by an arrow A. In the optical unit 10, a laser light beam generated by the semiconductor laser element 1 is diffracted by a diffraction grating 2 to be divided into three beams, which are represented with a single solid line in FIG. 1 for the sake of simplification, and each of the three laser light beams is directed to a polarizing beam splitter 3 to pass through a plane of analyzer 3a therein and then enters into a collimator lens 4. The laser light beam collimated by the collimator lens 4 passes through a quarter-wave plate 5 and enters into the objective lens 6 to be focused thereby to impinge upon the optical record disc D. On that occasion, three laser light beams are so aligned as to form a center beam used for reading an information signal from the spiral record track and detecting a focusing condition of each beam on the optical record disc D and two side beams positioned at both sides of the center beam to be used for detecting a tracking condition of the center beam in relation to the spiral record track on the optical record disc D. At the spiral record track on the optical record disc D, the three laser light beams are modulated in intensity and then reflected to form three reflected laser light beams.
The three reflected laser light beams from the optical record disc D return through the objective lens 6 to pass through the quarter-wave plate 5. Each of three reflected laser light beams having passed through the quarter-wave plate 5 has its plane of polarization rotated by ninety degrees in comparison with the respective laser light beam incident upon the optical record disc D.
The three reflected laser light beams having passed through the quarter-wave plate 5 pass through the collimator lens 4 into the polarizing beam splitter 3 and are reflected at the plane of analyzer 3a in the polarizing beam splitter 3 to be guided through a light receiving lens 7 to the photodetector 8. Then, detection output signals obtained based on a center beam and two side beams of three reflected laser light beams respectively are derived separately from the photodetector 8.
As shown in FIG. 2, a detecting portion 8A of the photodetector 8 provided for detecting the center beam is composed of four photodetecting elements 8a, 8b, 8c and 8d and a reproduced information signal and a focus error signal are obtained based on respective output signals of four photodetecting elements 8a to 8d. The photodetecting elements 8a to 8d are disposed close to one another and so arranged that the photodetecting elements 8a and 8b are aligned to cross at a substantially right angle to a direction corresponding to the tangential direction of the spiral record track on a plane in which the detecting portion 8A is disposed, and the photodetecting elements 8d and 8c are also aligned to cross at a substantially right angle to the direction corresponding to the tangential direction of the spiral record track on the plane in which the detecting portion 8A is disposed. The center beam reaching the detecting portion 8A forms a beam spot P on the photodetecting elements 8a to 8d thus arranged and the photodetecting elements 8a to 8d produce respective output signals Sa, Sb, Sc and Sd each corresponding to a portion of the beam spot P formed on each of the photodetecting elements 8a to 8d.
For producing the reproduced information signal, the output signals Sa, Sb, Sc and Sd obtained from the photodetecting elements 8a to 8d are summed up at an adder 9 to produce an added signal Se as shown in FIG. 2 and the added signal Se is used as the reproduced information signal, in the same manner as disclosed in, for example, U.S. Pat. No. 4,079,247.
In such a case, the information signal is recorded usually in the form of small pits arranged in the spiral record track on the optical record track, and therefore the center beam is subjected to diffraction caused in response to each of the pits by a relative movement of the spiral record track to the center beam irradiating the spiral record track resulted from the rotation of the optical record disc. Accordingly, in the beam spot P formed on the photodetecting elements 8a to 8d by the center beam, a diffraction pattern corresponding to each pit in the spiral record track moves, for example, in a direction indicated with an arrow T in FIG. 2, that is, a direction from the photodetecting element 8a to the photodetecting element 8d or from the photodetecting element 8b to the photodetecting element 8c, at a speed corresponding to a moving speed of the spiral record track, and each of the output signals Sa to Sd varies in accordance with variations in the diffraction pattern moving in the beam spot P. As a result, phase differences arise between the output signal Sa of the photodetecting element 8a and the output signal Sd of the photodetecting element 8d and between the output signal Sb of the photodetecting element 8b and the output signal Sc of the photodetecting element 8c, respectively, in response to the movement of the diffraction pattern in the beam spot P. The amount of each phase difference is determined by various factors including the moving speed of the spiral record track relative to the center beam irradiating the spiral record track, the size of the detecting portion 8A, the size of the beam spot P formed on the detecting portion 8A and so on. Consequently, in the case where such phase differences are not negligible, there is a problem that a proper reproduced information signal cannot be obtained by merely adding up the output signals Sa to Sd of the photodetecting elements 8a to 8d, respectively.
For the purpose of eliminating this problem, there has been proposed an improved signal processing circuit for producing a reproduced information signal based on the output signals Sa to Sd of the photodetecting elements 8a to 8d, as shown in FIG. 3.
In the signal processing circuit shown in FIG. 3, the output signals Sa and Sb derived from the photodetecting elements 8a and 8b, respectively, are added to each other in an adder 11 to produce an added signal Sa+Sb, and the added signal Sa+Sb is supplied to a phase shifter 13 which is operative to cause the added signal Sa+Sb to have a phase lag corresponding to the phase difference between the output signals Sa and Sd derived respectively from the photodetecting elements 8a and 8d, or between the output signals Sb and Sc derived respectively from the photodetecting elements 8b and 8c. Further, the output signals Sc and Sd derived respectively from the photodetecting elements 8c and 8d are added to each other in an adder 12 to produce an added signal Sc+Sd. Then, the added signal Sa+Sb subjected to the phase lag by the phase shifter 13 and the added signal Sc+Sd obtained from the adder 12 are added to each other in an adder 14 to produce a reproduced information signal Si. This circuit arrangement results in that the reproduced information signal Si is obtained by adding up the output signals Sa to Sd after each of the phase differences between the output signals Sa and Sd and between the output signals Sb and Sc is eliminated, so that waveform distortion of the reproduced information signal Si is reduced and a signal to noise ratio of the reproduced information signal Si is improved.
In the meantime, when an optical disc player employing the optical unit 10 shown in FIG. 1 and a signal processing circuit connected to the optical unit 10 is used for reproducing information signals from a double-faced optical record disc which has respective spiral record tracks at both sides, as shown in FIG. 4, the optical unit 10 is disposed to face a double-faced optical record disc D' loaded rotatably on a disc driving table 15 and, for example, first an information signal recorded on the side of a surface Da is reproduced and then another information signal recorded on the side of a surface Db is reproduced. On that occasion, after the reproduction of the information signal recorded on the side of the surface Da has been completed, the optical unit 10 which has faced the surface Da is moved through a path detouring around the double-faced optical record disc D' to face the surface Db for the reproduction of the information signal recorded on the side of the surface Db, as shown in FIG. 5.
In such a case, the double-faced optical record disc D' is kept continuously on the disc driving table 15 during both reproductions of the information signal recorded on the side of the surface Da and the information signal recorded on the side of the surface Db, but the direction of rotation of the double-faced optical record disc D' is changed to reverse between the reproduction of the information signal recorded on the side of the surface Da and the reproduction of the information signal recorded on the side of the surface Db. Therefore, the direction of movement of the spiral record track on the side of the surface Da in relation to the center beam from the optical unit 10 incident upon the surface Da and the direction of movement of the spiral record track on the side of the surface Db in relation to the center beam from the optical unit 10 incident upon the surface Db, are opposite to each other. This results in that each of the phase differences between the output signal Sa of the photodetecting element 8a and the output signal Sd of the photodetecting element 8d and between the output signal Sb of the photodetecting element 8b and the output signal Sc of the photodetecting element 8c, which arise in the signal processing circuit, is inverted between the reproduction of the information signal recorded on the side of the surface Da and the reproduction of the information signal recorded on the side of the surface Db, and consequently the improved signal processing circuit having the phase shifter 13 and shown in FIG. 3 is not available, so that waveform distortion of the reproduced information signal cannot be reduced and a signal to noise ratio of the reproduced information signal cannot be improved.
Such problems as mentioned above arise also on an optical disc player which employs the optical unit 10 shown in FIG. 1 and a signal processing circuit connected to the optical unit 10 and is constituted to able to reproduce selectively both information signals from optical record discs of two different types, such as a compatible optical disc player which is operative to reproduce selectively information signals from a video disc which has its diameter of approximately 30 cm and on which a television signal is recorded and a compact disc which has its diameter of approximately 12 cm and on which a digital audio signal is recorded.
As shown in FIG. 6, in the compatible optical disc player, a first disc driving table 21 on which a video disc VD is loaded and a second disc driving table 22 on which a compact disc CD is loaded are disposed separately and the optical unit 10 is provided between the first and second disc driving tables 21 and 22 to be movable linearly in a direction along an imaginary line passing through both of the centers of the first and second disc driving tables 21 and 22. The laser light beams emitted from the optical unit 10 to form including the center beam and the side beams are caused to impinge upon one of the video disc VD and the compact disc CD rotated by the first disc driving table 21 and the second disc driving table 22, respectively, so as to read the information signal recorded on the video disc VD or the compact disc CD.
When the laser light beams from the optical unit 10 trace a spiral record tracks formed on the video disc VD and the compact disc CD, the optical unit 10 moves linearly along an opening 23 provided between the first and second disc driving tables 21 and 22 in a predetermined direction from one to the other of the first and second disc driving tables 21 and 22. The optical unit 10 having such a linear movement between the first and second disc driving tables 21 and 22 moves in the radial direction from the innermost radius to the outermost radius of the video disc VD and in the radial direction from the outermost radius to the innermost radius of the compact disc CD, or in the radial direction from the innermost radius to the outermost radius of the compact disc CD and in the radial direction from the outermost radius to the innermost radius of the video disc VD. Further, the video disc VD and the compact disc CD are rotated in the same direction by the first and second disc driving tables 21 and 22, respectively.
Therefore, the direction of movement of the spiral record track on the video disc VD in relation to the center beam from the optical unit 10 incident upon the video disc VD and the direction of movement of the spiral record track on the compact disc CD in relation to the center beam from the optical unit 10 incident upon the compact disc CD, are opposite to each other. This results in that each of the phase differences between the output signal Sa of the photodetecting element 8a and the output signal Sd of the photodetecting element 8d and between the output signal Sb of the photodetecting element 8b and the output signal Sc of the photodetecting element 8c, which arise in the signal processing circuit, is inverted between the reproduction of the information signal recorded on the video disc VD and the reproduction of the information signal recorded on the compact disc CD, and consequently the improved signal processing circuit having the phase shifter 13 and shown in FIG. 3 is not available for the compatible optical disc player.